Observation of Electric-Dipole Transitions in the Laser-Cooling Candidate Th$^-$

Despite the fact that the laser cooling method is a well-established technique to obtain ultra-cold neutral atoms and atomic cations, it has so far never been applied to atomic anions due to the lack of suitable electric-dipole transitions. Efforts of more than a decade currently has La$^-$ as the only promising candidate for laser cooling. Our previous work [Tang et al., Phys. Rev. Lett. 123, 203002(2019)] showed that Th$^-$ is also a potential candidate. Here we report on a combination of experimental and theoretical studies to determine the relevant transition frequencies, transition rates, and branching ratios in Th$^-$. The resonant frequency of the laser cooling transition is determined to be $\nu/c$ = 4118.0 (10) cm$^{-1}$. The transition rate is calculated as A=1.17x10^4 s$^{-1}$. The branching fraction to dark states is very small, 1.47x10$^{-10}$, thus this represents an ideal closed cycle for laser cooling. Since Th has zero nuclear spin, it is an excellent candidate to be used to sympathetically cool antiprotons in a Penning trap.